I think it would be helpful to have a meta tag
tag-request in order to distinguish requests for new tags from other tag-related questions. Are there any reasons why this is hasn't been implemented?
It doesn't really need to be implemented.
The only users this affects are those with less than 300 reputation, and if they feel that their question needs a tag then they should first ask their question and apply whatever tags that are relevant and currently exist. They would then come to meta and request that their question is re-tagged with an extra tag that would have to be created.
Users with more than 300 reputation would just create the tag, thus not needing to request it in the first place.
A new tag should never be created without a question to attach it to. If a tag is created but not applied to any questions then it will be noticed by one of the nightly scripts and deleted, thus orphaning any wiki excerpts that might have been filled out.
As a result we've never needed tag requests without first having to retag a question first, hence we only need retag-request.